By H. Armon. Columbia Union College.
A user will generally not inspect the whole answer set A returned by an IR system—who would want to click through all of the thousands of hits typically returned by a web search? Therefore order chloromycetin 500 mg line symptoms iron deficiency, a much better performance description is given by the so-called precision-recall-curve 500 mg chloromycetin with amex medicine 0027 v. Here, the documents in A are sorted according to some ranking criterion and the precision is plotted against the recall values obtained when truncating the sorted answer set after a given rank—which is varied between one and the full size of A. Therefore, the quality of a search engine which generates hit lists is strongly dependent on its ranking algorithm. In essence, each document (or page) is weighted with the number of hyperlinks it receives from other web pages. At the same time the links themselves are weighed by the importance of the linking page (Brin & Page, 1998). The analogue to hyperlinks in the scientific literature are citations, that is, the more often an article is cited, the more important it is (Lawrence, Giles & Bollacker, 1999). The Role of Context So, we can handle unstructured text and measure how good an IR system works, but a critical question was not answered yet: How do we determine, which documents from a large collection are actually relevant to a given query? Copying or distributing in print or electronic forms without written permission of Idea Group Inc. So, the context—in the sense of the situation in which the user is immersed—plays a major role (Johnson, 2003; Lawrence, 2000). Budzik, Hammond and Birnbaum (2001) discuss two common approaches to handle different user contexts: 1. Relevance feedback: The user begins with a query and then evaluates the answer set. By providing positive or negative feedback to the IR system, this can modify the original query by adding positive or negative search terms to it. In an iterative dialogue with the IR system, the answer set is then gradually narrowed down to the relevant result set. However, as studies (Hearst, 1999) have shown, users are generally reluctant to give exhaustive feedback to the system. Building user profiles: Similar to the relevance feedback, the IR system builds up a user profile across multiple retrieval sessions, that is, with each document the user selects for viewing, the profile is adapted. Unfortunately, such a system does not take account of “false positives”, that is, when a user follows a link that turned out to be of no value when inspecting it closer. Additionally, such systems integrate short term user interests into accumulated context profiles, and tend to inhibit highly specialized queries which the user is currently interested in. Budzik, Hammond & Birnbaum (2001) presented a system that tries to guess the user context from open documents currently edited or browsed on the work space. In an evaluation of their system both achieved consistently better results than standard search engines are able to achieve without context. Additionally, the user might discover a context he had not in mind when formulating the query and thus find links between his intended and an Copyright © 2005, Idea Group Inc. Copying or distributing in print or electronic forms without written permission of Idea Group Inc. Interactive Information Retrieval Towards Effective Knowledge Management 61 unanticipated context. Augmenting Document Sets with Context Text Categorization One way to add context to a document is by assigning a meaningful label to it (Le & Thoma, 2003). This constitutes a task of text categorization and there exist numerous algorithms that can be applied. The general approach is to select a training set of documents that are already labelled. Based on the “bag of words” representation, machine learning methods learn the association of category labels to documents. For an in depth review of statistical approaches (such as naives Bayes or decision trees) see Yang (1999).
His-Purkinje System and Ventricular Muscle The most common are dizziness or light-headedness chloromycetin 250mg with mastercard symptoms kidney disease, Propafenone slows conduction and inhibits auto- metallic taste cheap 500 mg chloromycetin fast delivery symptoms 5 days after conception, nausea, and vomiting; the most serious matic foci. Electrocardiographic Changes Contraindications Propafenone causes dose-dependent increases in the Propafenone is contraindicated in the presence of se- PR and QRS intervals. Other contraindications include by an increase in right atrial, pulmonary arterial, and severe bradycardia, hypotension, obstructive pulmo- pulmonary artery wedge pressures in addition to an in- nary disease, and hepatic and renal failure. Because of crease in vascular resistance and a decrease in the car- its weak -blocking action, propafenone may cause pos- diac index. This problem is great- may be observed in patients with preexisting left ven- est in patients who are slow metabolizers. In the absence of cardiac abnor- malities, propafenone has no signiﬁcant effects on car- diac function. SA, sinoatrial; D, decrease in conduction velocity; I, increase in conduction velocity; –, no signiﬁcant effect with clinically relevant doses;,mini- mal effect. Bear in mind the com- Atrium plete spectrum of cardiovascular effects of these agents Propranolol has local anesthetic properties and ex- when prescribing their use. For example, while patients erts actions similar to those of quinidine on the atrial with a normally functioning cardiovascular system may membrane action potential. Membrane responsiveness tolerate adrenergic blockade of the heart, patients with and action potential amplitude are reduced, and ex- compensated heart failure, who depend on adrenergic citability is decreased; conduction velocity is reduced. A-V Node Propranolol The depressant effects of propranolol on the A-V node are more pronounced than are the direct depres- Propranolol (Inderal) is the prototype -blocker (see sant effects of quinidine. It decreases the effects of sympathetic dual actions of -blockade and direct myocardial de- stimulation by competitive binding to -adrenoceptors. Propranolol administration results in a de- crease in A-V conduction velocity and an increase in Electrophysiological Actions the A-V nodal refractory period. The second is associated with its direct myo- His-Purkinje System and Ventricular Muscle cardial effects (membrane stabilization). The latter ac- Propranolol decreases Purkinje ﬁber membrane re- tion, especially at high clinically employed doses, may sponsiveness and reduces action potential amplitude. These which enhanced -receptor stimulation does not play a changes result in a decrease in His-Purkinje conduction signiﬁcant role in the genesis of the rhythm disturbance. However, these electrophysiological alter- ations are observed at propranolol concentrations in Sinoatrial Node excess of those normally used in therapy. The most Propranolol slows the spontaneous ﬁring rate of striking electrophysiological property of propranolol nodal cells by decreasing the slope of phase 4 depolar- at usual therapeutic concentrations is a depression of ization. Blockade of -receptors Individual Antiarrhythmic prolongs systolic ejection periods at rest and during ex- Drugs ercise. However, these alterations are Drugs Therapeutic Uses offset by factors that tend to reduce oxygen consump- tion, such as decreased heart rate and decreased force Acebutolol Ventricular arrhythmias, ventricular ectopy of contraction. The decrease in oxygen demand pro- Adenosine Supraventricular tachycardia duced by a decrease in heart rate and a decrease in Wolff-Parkinson-White syndrome force of contraction is usually greater than the increase Amiodarone Hemodynamically unstable ventricular in oxygen demand that results from increased heart size tachycardia and increased ejection time. The net result is that oxy- Ventricular ﬁbrillation Bretylium Ventricular arrhythmias after cardiac gen demand is decreased. In se- Procainamide Atrial tachycardia, ventricular lected cases of sinus tachycardia caused by anxiety, tachycardia Premature ventricular contractions pheochromocytoma, or thyrotoxicosis, -blockade will Propafenone Atrial ﬁbrillation, ventricular reduce the spontaneous heart rate. Patients with Postoperative ventricular arrhythmias Wolff-Parkinson-White syndrome supraventricular extrasystoles and intermittent parox- Quinidine Atrial arrhythmias, ventricular ysms of atrial ﬁbrillation may beneﬁt from -receptor tachycardia blockade with propranolol. Sotalol Ventricular arrhythmias, ventricular The arrhythmias associated with halothane or cyclo- ﬁbrillation propane anesthesia have been attributed to the interac- Tocainide Premature ventricular contractions Ventricular tachycardia tion of the anesthetic with catecholamines, and they Verapamil Paroxysmal supraventricular tachycardia have been suppressed by IV administration of 1 to 3 mg Atrial ﬁbrillation propranolol. An increase in circulating catecholamines also has been observed in patients with acute myo- cardial infarction and has been correlated with the de- velopment of arrhythmias. Electrocardiographic Changes Clinically, tachyarrhythmias associated with digitalis excess (including supraventricular and ventricular ex- Propranolol prolongs the PR interval but does not trasystoles) and ventricular tachycardia have been sup- change the QRS interval. Although propranolol is highly effective in the treatment of digitalis-induced arrhyth- Hemodynamic Effects mias, phenytoin and lidocaine are preferred.
In patients with limb amputation or denervation discount chloromycetin 500 mg symptoms 4dpo, the activity of the corresponding part of the motor cortex tends to decrease over time discount 500 mg chloromycetin amex medications 6 rights. At the same time, the patients may experience subjective phenomena such as phantom limb or phantom pain. As already mentioned in the section on cortical plasticity, Giraux and Sirigu,9 in two such patients with a unilateral brachial plexus avulsion, showed that a normal activity could be restored in their motor cortices by means of observational training. These patients were trained in a situation where the image of the valid hand was visually transposed (by way of mirrors; see Reference 69) at the location of the impaired hand. They were instructed to move the valid hand that they could see at the place of the impaired arm. Following a series of 24 daily visuomotor training sessions, the activity of the M1 area contralateral to the impaired (paralyzed) arm, monitored by fMRI, was greatly increased. This result shows that observation of a normally moving limb can activate the motor area controlling the homologous limb in the observer. A possible explanation for this phenomenon is that mirror neurons similar to those found in the monkey premotor cortex are activated by observation of the Copyright © 2005 CRC Press LLC moving hand and trigger the activity of M1 neurons innervating the impaired hand. In these two patients, this visuomotor training procedure had a beneﬁcial effect, not in improving their hand movements, which remained impaired due to the plexus brachial lesion, but in decreasing their phantom limb pain. This effect suggests that the motor cortex, in addition to its role in controlling movements, may also inﬂuence the processing of sensory input arising from the effector it controls. The ability of human subjects to imitate each other derives from the possibility of forming action representations from the observation of other people. According to this hypothesis, brain areas that become active during execution of a movement should become more active when that movement is elicited by the observation of an identical movement made by another person. Areas in the left inferior frontal cortex and in the parietal cortex seem to fulﬁll this requirement. This result, showing that brain areas for execution and observation of a movement are matched, is compatible with the simulation theory. Furthermore, it raises again the point (already raised in the section about shared representations) of how two different representations of the same action are disen- tangled from one another. Other studies dealing with imitation71 suggest that areas might be activated differently whether an action is internally produced or generated by another agent. First, because all aspects of action appear to be involved during action repre- sentation, it seems a logical consequence of this rehearsal of the corresponding brain structures, and speciﬁcally the motor structures, that the subsequent execution will be facilitated. The presence of activity in the motor system during covert action puts the action representation in a true motor format, so that it can be regarded by the motor system as a real action. In addition, imitation seems to be based on directly matching the observed action onto an internal simulation of that action. Second, activation of the motor cortex and of the descending motor pathway seems to fulﬁll several critical functions. First, this activation contributes to gener- ating corollary signals that propagate upstream to the parietal and premotor cortices. This mechanism would allow for evaluating the potential consequences of the future action. The well-known “antisaccade” task and its analogues in reaching serve as special cases of such transformational mapping, one form of nonstandard mapping. Other forms of nonstandard mapping differ from both of the above: they are arbitrary. Here we explore several types of arbitrary mapping, with emphasis on the neural basis of learning these behaviors. Like braking at a red trafﬁc light, Pavlovian learning depends on an arbitrary Copyright © 2005 CRC Press LLC relationship between a response and the stimulus that triggers it. The example of braking at a red light, but accelerating at a yellow one, serves as a prototypical (and sometimes dangerous) example of such behavior. In the laboratory, this kind of task goes by several names, including conditional motor learning, conditional discrimination, and stimulus–response conditioning.
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